System for defrost control of refrigerators and the like



March 19, 1968 J. K. NELSON 3,373,575

SYSTEM FOR DEFROST CONTROL OF REFRIGERATORS AND THE LIKE Original Filed Aug. 29, 1966 FIGJ L I? LI F I G. 4

INVENTOR JAMES K. NELSON 2 Sheets-Sheet 1 I SYSTEM FOR DEFROST CONTROL OF REFRIGERATORS AND THE LIKE 2 Sheets-Sheet 2 Original Filed Aug. 29, 1966 FIG.2

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2 5 M 15 HK $2 28L 0 02 9 6 2\ 2 .2.! J| 2 L l 4 .w FIJI 3 INVENTOR JAMES K. NELSON BY ATTY.

United States Patent G 3,373,575 SYSTEM FGR DEFRQST CONTROL OF REFREGERATORS AND THE LlKE James K. Nelson, Des Plaines, lllL, assignor to Borg- Warner Corporation, a corporation of Illinois Continuation of abandoned application Ser. No. 575,648, Aug. 29, 1966. This application Jan. 9, 1967, Ser. No.

8 Ciaims. (Cl. 6280) ABSTRACT OF THE DESCLOSURE This is a continuation of application Ser. No. 575,648 filed Aug. 29, 1966, now abandoned.

The ,present invention relates to a procedure and apparatus for defrosting, and more particularly to improved automatic defrosting of refrigerators, freezers and the like.

A problem encountered in present-day refrigerators, freezers, or the like is the automatic removal of frost from cooled surfaces. For example, in the evaporator area of a refrigerator or freezer the low temperatures provide a surface on which moisture entering the refrigerator via warm air from a room condenses, forming frost thereon. If the layer of frost on an evaporator is relatively thick, the cooling efficiency of the latter is reduced. The frost can be more easily removed if it is sensed before it accumulates to a substantial thickness.

Though automatic controls have been developed, these on the whole, have been expensive and are not adapted to sense relatively thin layers of frost. In these known devices, because a substantial frost thickness must accumulate before the sensing apparatus is actuated, defrosting takes a longer time, increasing the heat input to the refrigerator and reducing the over-all efficiency of the unit.

Also, there are problems encountered in sensing thin layers of frost on road surfaces, airfoil surfaces and the like. The sensing of such frost to operate suitable indicating devices requires a particularly sensitive unit.

Accordingly, it is an object of the present invention to provide an improved apparatus and method for operating a defrost system of a refrigerator, freezer, or the like which can sense comparatively thin accumulations of froit on a surface and automatically effect removal thereof. The present invention will find advantageous but by no means exclusive use in a refrigerator to sense accumulation of a thin layer of frost in the interior, such as in the area of an evaporator, and respond by effecting shortperiod operation of a defrosting system, for example of a hot gas or an electrical heater type, thereby melting the frost.

It is a more detailed object of the present invention to provide an improved apparatus and method which utilizes a relatively warm element to melt accumulated frost and subsequently, freezing the water formed, to effect adherice? once of the element to the frosted surface to indicate excess formation of frost.

It is yet another object of the present invention to provide an improved apparatus and method of the above type for controlling defrosting which includes means for terminating the defrost operation after a predetermined period of time.

It is an over-all object of the present invention to provide a defrost system with improved controls usable in a refrigerator, freezer or the like, which is economical to manufacture and easy to maintain.

Other objects and advantages of the present invention will become apparent upon reading the following description and upon reference to the drawings in which:

FIGURE 1 is a schematic of a control system embodyin g the present invention;

FIGURE 2 is an elevation a frost sensing structure for FIGURE 1;

FIGURE 3 is a section taken along line 3-3 of PEG- URE 2; and

FIGURE 4 is an alternative embodiment of the frost sensing means of FIGURE 2.

Turning to the drawings and an exemplary embodiment of the present invention, shown in FIGURE 1 is an electrical schematic diagram 10 of an exemplary portion of a refrigerating system energized from a source represented by lines L1, L2, respectively. Describing the exemplary refrigerating system, the latter includes a compressor motor 11 operative to effect cooling of a refrigerator enclosure (not shown) by pumping refrigerant through an evaporator area in the refrigerator. A fan motor 12 effects circulation of air within the refrigerated area to facilitate maintenance of a predetermined temperature. For controlling energization of compressor motor 11, a thermostat 14, responsive to temperature in the cooled enclosure, effects operation of a switch 15 which completes an energizing circuit between source lines L1, L2. To defrost a surface or area within the cooled interior, a defrost heater 16 is connected in series with a switch 18 across the sources L1, L2. The defrost heater 16 is positioned in proximity with an evaporator or other surfaces within a refrigerator, freezer or the like, which tend to collect frost, so that operation of the heater effects defrosting of that particular surface or area.

In accordance with the present invention, an improved defrost control is provided for sensing relatively thin accumulations of frost on a surface representative of frost collecting surfaces within the interior of a refrigerator, freezer or the like, and responsive to such frost accumulation or collection to initiate operation of defrost means, in the present instance the eater 16. As herein illustrated, a defrost control assembly 20 is mounted in proximity to a refrigerator evaporator tube 21 representing an evaporator area, to sense accumulation of frost 22a on a representative surface 22 (see FIGURES 2, 3). The representative surface is illustratively shown as one side of a metal block 24 constructed, for example, of heat conductive material such as aluminum, copper or the like. The temperature of the surface 22 substantially corresponds to that of the evaporator tube 21. Thus, if moisture has condensed in the evaporator environment, there is also accumulation of frost on the surface 22. To assure that moisture laden air comes in contact with the representative surface, apertures 23 are provided in the control assembly enclosure.

For sensing the accumulation of frost on the surface 22, actuatable means, herein illustrated as a probe 25,

partially in section showing operating the switches of is provided with an end portion 26 having a surface 26a which is reciprocally moved toward and away from the sensed or representative surface 22. The probe surface 26a is warmed by a heater 28 to melt the frost that is formed in the proximate area of the probe surface 25a, illustratively shown as a gap 29. The probe moves relatively slowly, so that by lingering after melting the frost, it is cooled. Furthermore, the material of which block 24 is formed has high thermal conductivity, thereby removing the heat in the water. As a result if there has been sufficient accumulation of frost so that upon melting the gap 29 is filled with water, the portion 26 of the probe is frozen or made to adhere to the representative surface 22. For selecting a thickness of frost which results in the described adherence, means are provided for adjusting the width or thickness of space 29. As herein illustrated, a screw 30 formed, for example, of plastic or other like material having a low thermal conductivity as compared to the probe, is received in a tapped aperture in the probe portion 26. The contacting screw 30 could also be formed with a relatively small contacting surface, such as a pin point. In that instance, the thermal conductivity of the screw material could be the same as that of the probe. The screw 30 can be turned in or out to respectively increase or decrease the proximity to which probe surface 26:: can be brought before sticking or adherence to the sensed surface 22 occurs. Of course, other means limit the proximity with which the probe surface 26a can be brought against the representative surface 22. These means could be of the stop-limit type well known to those skilled in the art.

For reciprocally moving the sensor or probe 25 toward and away from the representative or sensed surface 22, a power means is provided, in the present instance a bimetal system 31. It is of course understood that other power units providing cyclical movement, such as a timing motor or the like can also be used. The bimetal system 31 includes a flexible bimetal member 32 carried on a mounting block 34 which is illustratively supported in a body of insulation material 35. The latter can be manufactured of polystyrene foam, for example, which provides insulation as well as support for the mounting block. For heating the bimetal, an element 36 is carried within the block 34. The size of block 34 is selected, so as to delay the transmittal of heat to bimetal member 32.

As viewed in FIGURE 2, a lower end 38 of sensing probe 25 is secured to a middle portion 37 of bimetal member 32. For effecting reciprocal movement of the bimetal 32 and thereby the probe 25, a cycle switch 39 is provided. The cycle switch includes an actuation button 46 which is positioned so as to be pressed by the bimetal member 32 and thereby open switch contacts 41 when the bimetal member 32 moves to an extreme left position upon being heated (position shown in bold lines in FIGURE 2). Of course, when the energy is not supplied to the heater element 36, the bimetal cools and moves to the right as viewed in FIGURE 2, thereby deactuating switch button 40 and closing switch contacts 41 (position shown in phantom FIGURE 2). The result is that energy is again supplied to heater 36, and the cyclical movement to probe 25 is started again. The probe heater 28 is energized simultaneously with energization of the heater element 36. Thus, the probe surface is warmed while the bimetal member is moving to the left as viewed in FIG- URE 2. On reaching its extreme left position, if the probe surface 26 encounters a sufficient build-up of frost, the latter is melted and a globule of water is formed in the space 29.

To actuate operation of the defrost means, herein illustrated as a defrost heater 16, and also to coordinate operation of the fan motor 12 so that the latter does not circulate air while defrosting is occurring in the evaporator environment, a pair of switches-a defrost actuating switch 42 and a fan actuating switch 43 are provided. These are carried on an end portion 44 of the bimetal member 32.

The defrost system switch 42 effects closing of contacts 18, when the probe portion 25 adheres to the frost collecting surface 22, to thereby energize defrost heater 16. To this end, the probe 25 is flexible in a central portion 45 so that the bimetal is permitted to move to the right, as a part of its cyclical movement, even though the upper end of the probe is frozen to block 24. The switch 42 has an actuatable button 46 which engages probe portion 45, resulting in closing of contacts 18, thereby completing an energization circuit for defrosting means 16.

For energizing the fan motor 12 during normal operation of the refrigerator, while deenergizing the fan motor during defrosting, the fan switch includes a pair of 0perative buttons 48, 49. The switch 43 is also carried on the end portion 44 of the bimetal in the same manner as the switch 42. The button 48 is also positioned adjacent the central portion 45 of the probe 25 so as to be actuated by the movement of bimetal 32 to the right when the probe is held stationary at its upper end. The operation of button 48 effects opening of a set of switch contacts 50. The switch contacts 53 remain open so that the fan motor 12 is not energized until the reset button 49 for the fan switch is operated. This occurs when the bimetal 32 is heated and returns to the left, as viewed in FIGURE 2, as a part of its cyclical movement. To engage the reset button 49, a bracket 51 is mounted on an inside wall of the control assembly housing and is provided with a flat portion 52 positioned to press the reset button 49 when the bimetal 32 carries the switch to its extreme left position.

Summarizing, operation of the present device, if frost has accumulated in the space 29 between the frost collecting surface 22 and the probe sensing surface 26a, the warm probe surface melts the frost. Because the block 24 is maintained at substantially the temperature of evaporator coil 21, which is below freezing, the heat in the water is absorbed or conducted away, and a frozen globule is formed. If the globule fills the width or thickness of the gap 29, adherence or sticking of the probe surface to the sensed or representative surface 22 is effected. If there is not sufiicient water formed so that upon freezing the gap 29 is not bridged, then the surface 26 of the probe does not stick to the block surface 22.

As a part of continual cyclical operation, the bimetal 52 is heated and then permitted to cool so as to effect movement of the probe 25-during heating, movement is to the left as viewed in FIGURE 2 and during cooling, movement is to the right. The insulation, the physical characteristics of mounting block 34, heater 36 and bimetal 32 determines the time period of a cycle of operation of the bimetal, i.e., movement from one extreme position to the opposite extreme position and then a return to the original position. In one practical instance a cycle of 30 minutes was selected.

If the probe sticks or is frozen to the block 24, then, as the bimetal 32 cools and carries the respective switches 42 and 43 to the right (as viewed in FIGURE 2) the respective defrost operating switch and the fan motor deenergizing switch are operated. This effects heating of the evaporator environment so as to remove frost. At the same time, the representative surface 22 is heated. When the probe is unstuck, defrosting is terminated. This is an additional feature of the control shown herein, i.e., means are provided for automatically terminating defrosting eliminating the necessity for accessory controls, such as a thermostat, timer or the like to end the defrost cycle.

It takes varying periods of time to completely defrost an evaporator or like frost collecting surface because of the non-uniformity of collection of frost and also because of non-uniformity in the application of defrosting heat. Since the representative surface may not be the last to defrost, it may be necessary to design a lag into the system to assure complete removal of frost. To achieve this, a lag reservoir 54 may be provided in the block 24, to delay heating of the sensed representative surface 22.

In this manner, the period of time before the probe becomes unstuck is extended, thereby permitting completion of defrosting.

Of course, in the foregoing description of operation, if the probe does not stick then the normal operation of the refrigerator, freezer or the like is not affected. Under those conditions the cycle is repeated and defrosting does not occur until the frost accumulates to a point resulting in the probe sticking to the representative sensed surface.

It is another feature of the present inventive defrosting system, that is of particular importance in a gas refrigerator. In the latter when the room ambient temperature is at a low value, below about 60 F., the defrosting ability of a hot gas defrosting system is at a low level. It is preferable if the defrost system is turned on during this time, to have it turn off and defrosting be attempted at a later more favorable time when the room temperature is higher. To this end, the probe becomes unstuck when it is heated and effects termination of the defrosting operation. This is a built-in time-override.

The steps in the operation of the present invention are first, the representative surface 22 is sensed for frost by the probe 25. In addition, the probe is heated so that if a suflicient accumulation of frost is sensed, the frost is melted, forming a globule of water in the space 29. Subsequently, the water is cooled by the low temperature of block 24 forming ice which causes the probe to adhere to surface 22. Finally, the adherence or freezing of the probe 25 to the surface 22 is used to operate the defrosting means 16. This effects removal of frost from the evaporator or other frost collecting structure within a refrigerator, freezer or the like.

Shown in FIGURE 4 is an alternative embodiment of a defrost system control; the structure differs in the operative means provided for effecting cycling of the probe 25. The bimetal or cycling system 60 includes a bimetal member 61 carrying a heat conducting and magnetic force responsive member 62. The bimetal is secured at its lower end 64 to a support 65 with the member 62 carried above the lower end so as to move in respective opposite directions as the bimetal member 62 flexes.

Disposed on either side of the member 62 are a pair of magnets 68, 69. Magnet 68 is mounted on the evaporator tube 21 and functions as a cold magnet. The magnet 69 is carried on the heater 36 and functions as a warm magnet. The bimetal 61, upon cooling, moves to the right as viewed in FIGURE 4. As the steel mass 62 comes under the influence of the flux of magnet 69, the member 62 is attracted thereto in a snap movement. Once the member 62 enters the flux field established by magnet 69, any movement toward the magnet has a multiple effect on the magnetic force. This is because of the well known inverse square relationship between distance and magnetic force. While the member 62 remains in contact with the magnet 69, it receives heat from the magnet which itself is heated by heater element 36. Thereby heat is conducted to the bimetal member 64 causing it to flex.

When the member 62 enters the magnetic field of the other magnet 68, again because of the inverse square relationship of magnetic field strength to distance from the magnet, the member 62 is snapped against the cold magnet 68. A cycle is started over again as the member 62 is cooled, thereby lowering the temperature of the bimetal member 61.

The alternative embodiment does not require a switch for intermittently energizing the bimetal heater 36. This is clear from the foregoing in that the snap movement effected by the magnets provides the switching over from cooling to heating, or vice verse, of the bimetal member 62.

Though in the preferred embodiment it has been assumed that a globule of water is provided in the space 29 between the probe surface 26a and the sensed surface 22,

it is understood that a very thin layer of water between the two surfaces will effect adherence. In that instance, the probe and the surface sensed are in sticking contact. Thus, very thin layers of frost can be sensed to effect operation of the defrosting means. Furthermore, though the illustrative embodiment describes defrosting of an evaporator environment, it is within the teachings of the present invention to defrost walls of the freezers and other areas in a refrigerated interior which collect frost. Accordingly, the present frost accumulation sensing device operates efficiently to effect energization of defrosting means in a variety of refrigeration installations. In the same vein though the embodiment of the inventive frost sensing unit is shown in an environment for operating a defrosting system, it is also usable to effect operation of other coupled devices, for example an airfoil ice remover or a road sign indicating formation of ice on a roadway.

While the invention has been described in connection with a preferred embodiment, it is understood that I do not intend to limit the invention to that embodiment. On the contrary, I intend to cover the alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

I claim as my invention:

1. In a system for defrosting a refrigerator or the like having a surface or environment customarily accumulating frost, a control for such system comprising in combination a cyclically operated probe adapted to sense accumulation of frost on a preselected surface, means for heating said probe for effecting melting of said sensed frost to form water, and heat conducting means for removing heat from, and thereby freezing said Water to effect sticking of said probe to said sensed surface, said probe melting sufficient frost for effecting adherence of said probe to said surface only when frost accumulates to a predetermined thickness on said preselected surface, and means responsive to sticking of said cyclically operated probe to effect operation of the defrost system and thereby remove the frost accumulation on the surface.

2. The combination of claim 1 including bimetal means operatively engaging said probe, a heater for actuating said bimetal for cycling operation of said probe and switch means actuated by the movement of said bimetal to energize and deenergize said heater.

3. The combination of claim 2 wherein said means for sensing sticking of said probe includes a switch having operative structure carried by said bimetal engageable with said probe which is maintained substantially stationary to thereby close said switch and energize said defrosting system.

4. The combination of claim 1 including adjustable means for selecting the proximity with which said probe can be moved toward said preselected surface thereby selecting the thickness of frost which effects probe sticking.

5. The combination of claim 1 including means responsive to release of said probe to terminate a defrosting cycle.

6. A method of automatically operating a defrost system, for example in a refrigerator, freezer or the like, to maintain a surface frost-free, comprising the steps of sensing a representative portion of the surface with a probe for a predetermined thickness of frost accumulation, heating said probe to melt said frost upon accumulation of the latter to said predetermined thickness, freezing said water to effect adherence of said probe to the surface, and sens ing the adherence of said probe to the surface to effect operation of the defrost system thereby automatically removing said frost.

7. The method of claim 6 including the steps of terminating operation of the defrost system upon completion of frost removal.

8. In a system responsive to the accumulation of frost comprising in combination a cyclically operated probe adapted to sense accumulation of frost on a preselected surface, means for heating said probe for effecting melting of said sensed frost to form water, and heat conducting means for removing heat from, and thereby freezing said Water to elfect sticking of said probe to said sensed surface, said probe melting suificient frost for effecting adherence of said probe to said surface only when frost accumulates to a predetermined thickness on said preselected surface, and means responsive to sticking of said cyclically operated probe to efiect operation of a coupled device.

References Cited UNITED STATES PATENTS 1/1950 Idrac 340Z34 3/1955 Morton 62-14O ROBERT A. OLEARY, Primary Examiner.

W. E. WAYNER, Assistant Examiner. 

